A Los Angeles school shooting is believed to have been caused by a semi-automatic handgun that went off inside a 12-year-old girl's backpack, police said.

The single round struck a 15-year-old boy in the temple and a 15-year-old girl in the wrist. Two other students and an adult suffered minor injuries from flying glass, according to authorities.

The boy who was shot in the head remains in critical condition and the girl injured in the arm is in stable condition, police said.

The 12-year-old was taken into custody after Thursday's incident at Sal Castro Middle School. She is being held at juvenile hall and faces charges of negligent discharge of a firearm and bringing a loaded firearm onto a school campus, police said.

Investigators are trying to determine how she obtained the unregistered weapon, according to LAPD Deputy Chief Robert Arcos, who spoke to reporters Friday.

The child is represented by an attorney and has not spoken to detectives, he said.

"We would love to be able to talk to her more fully in an interview and just get to the intent and motive," Arcos said. "There's a lot of things I'm sure are going through her mind right now — she's only 12 years old. Hopefully we'll have an opportunity again to talk to her."

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